Figure 3: a) FT-IR spectra of the four NSs. b) Water contact angle of natural nut shells (NNS) and treated nut shells (NSs). c) Photograph of i) natural WS, ii) treated WS, and iii) nano-engineered WS-H+. Confocal Laser Scanning Microscopy (CLSM) of the d) untreated natural walnut shell (NWS), e) treated WS and f) WS-H+ indicating the lignin reductions (color changes from green to blue). SEM image of the g) porous, thin layer of inner shell surface of WS-H+, h) nanostructured outer shell surface of WS-H+ indicating the random pore diameter of 78 nm and 190 nm, i) WS-H+ after dipping in alkaline reservoir (pH 12.5) for 8 h with a random pore diameter of 160 nm.
The CLSM images of Figure 3 (d, e, f) confirms the variations in lignin contents in WS. The lignin (green) tends to disappear after undergoing the chemical treatments, and more cellulose (blue) gets exposed, which indicates the partial delignification of the WS. SEM images of WS-H+, shown in Figure 3 (h) confirms the enhanced number of nanopores after the nanoengineering of the WS. A prominently porous layer on the interior surface was also observed in Figure 3 (g). Figure 3 (i) confirms the structural changes after dipping into the alkaline reservoir for 8 h, whereas the nanopores sustained.

Electricity generation and performance analyses

Herein, water droplet evaporation-based hydrovoltaic electricity generation has been investigated utilizing the nutshells. A sandwich structure of graphite-nutshell-graphite (G-NS-G) was utilized for the Open Circuit Voltage (Voc), Short Circuit Current (Isc), and Short Circuit Current density (Jsc) analyses. The typical porous NS structure, as schematically illustrated in Figure 4 (a), facilitates the water flow through the micro/nanochannels with multiple functional groups. When the DI water contacts this porous structure with abundant hydrophilic functional groups, water droplets are adsorbed and transferred through the micro/nanochannels under capillary action. At the same time, this abundant oxygenated functional group of the NS surface interacts with water and generates negative surface charges. water is freely transferred throughout these channels under capillary actions and the continual evaporation helps to circulate the water in the direction of water evaporation. The transportation duration of water from the bottom surface to the top surface is different among these four NS, which is depicted in Figure 4 (b). A very porous feature of AS along its surface and cross sections, confirmed byFigure-S 3 (a) causes the faster movement of water compared to other NSs.